1. Field of the Invention
The present invention relates to a muffler to be used for the exhaust system of an engine, for example, for automobiles, and a method of manufacturing the same.
2. Description of the Related Art
Hitherto, a muffler to be used for the exhaust system of an engine for automobiles or the like such as a sub muffler to be disposed immediately behind the catalytic converter, a main muffler to be placed at the rear of the exhaust system, or a sound absorbing muffler to be disposed in the main muffler, having a sound absorbing material interposed between the internal tube and the external tube is known.
However, in the muffler described above, the sound absorbing material may lean to one side or be deformed after long time of use due to vibration of the vehicle body, vibration of the engine, or expansion and contraction caused by exhaust heat. Especially, when the axial length of the sound absorbing material interposed between the external and the internal tubes is long, it tends to lean to one side, and when different types of sound absorbing materials multilayered in the direction of thickness tend to be subject to slippage or wrinkles at the mating portion. As a consequent, the sound absorbing material may vary in bulk density and thus lower the sound-muffling capability, or may scatter and thus lowering the durability disadvantageously.
Generally, the muffler as described above is manufactured in such a manner that the sound absorbing material is filled between the internal tube and the external tube, and then the end portion of the external tube is drawn into a tapered shape. In this case, the sound absorbing material is secured by a jig or the like so as not to be displaced in the related art. However, when the diameter of the tapered end decreases, the structure of the jig becomes complex, and there may be cases where it cannot be secured by a jig.